Hi
You have to create a workflow, go to settings==>workflow ==> create a new workflow for the email.
In the Entities select "email" and create a new workflow rule. Here configure the workflow like this.
Event:
Record Attribute Status Change (Select Activity Status)
Rules:
Insert Check Condition
If Activity Status == Email Sent && Email.From== "FromUser" then
Send Email (create new message). // Fill in the properties with dynamic values.
The above may create loop as you are sending email, so make sure the "From" for the 1st and 2nd email is different.
Note: The same can be achieve using a custom plugin (in C#) by creating a plugin against SetState & SetStateDynamic message on the email entity.
Muhammad Ali Khan
http://malikhan.wordpress.com